1. Field of the Invention
The present invention relates to a honeycomb structure.
2. Description of the Related Art
A honeycomb catalyst for converting automotive exhaust gases comprises a honeycomb structure made of cordierite, a layer formed on the surface of the honeycomb structure and made of a material such as activated alumina having a large specific surface area, and a catalyst such as platinum supported on the layer. And, a honeycomb catalyst for converting exhaust gases emitted from a diesel engine further includes a NOx-occluding agent to treat NOx in an excess oxygen atmosphere.
One problem with such a honeycomb catalyst is that a NOx-occluding agent tends to more stably occlude SOx rather than NOx and to become unable to properly occlude NOx because of SOx poisoning.
Trying to solve this problem, an apparatus disclosed in JP-A-6-58138 comprises a sulfur-capturing device positioned upstream of a NOx-occluding agent in the exhaust gas passage and including a sulfur sorbent and a casing surrounding the sulfur sorbent. An exemplary sulfur sorbent described in JP-A-6-58138 comprises an alumina support carrying a noble metal such as platinum and at least one of an alkali metal such as potassium, sodium, lithium, or cesium, an alkaline earth such as barium or calcium, and a rare earth such as lanthanum or yttrium. WO05/063653 also discloses a honeycomb structure.
The entire contents of JP-A-6-58138 and WO05/063653 are incorporated herein by reference.